I once took a bus from Lucca to Florence, and it sat for 20 minutes in a traffic jam on the autostrada. A train will be more reliable, and at least you have room to move around. There are trains at least once an hour, costing €7.60.
It is also possible to get from Rome to Pisa by train near the west coast, changing trains at Pisa. See www.trenitalia.com/tcom-en looking for trains from Roma Termini to Lucca. Use a travel date before 10 June, as local trains are not yet online past that date.
I second the train suggestion (but I will declare an aversion to buses).
The long distance trains in Italy in March/April were fabulous - with reserved seating you have space, comfort, toilets on the trip and stations were easy to navigate. You can book them directly on the Trentalia website.
amymk104, this might be of interest. The train station at Lucca is situated just outside the walls on Piazzale Ricasoli, when you exit the station head towards the walls of the city, bear left, cross the main ring-road (there is a pedestrian crossing) and enter the city through the Porta San Pietro.